Revised Abstract: This application is in response to program announcement PA-10-209 Biology of Manual Therapies. The NIH has recognized that low back pain (LBP) is a significant health problem and that there is a tremendous lack of mechanistic studies on one of the most popular treatments for this disorder (i.e., manipulation treatment). Back pain has a staggeringly negative impact on our society in terms of medical expenses, disability, and individual suffering. According to a recent national survey, more than 18 million Americans over the age of 18 years received manipulative therapies in 2007 at a total annual out of pocket cost of $3.9 billion. Spinal manipulative treatments can be broadly classified as manipulation-based or mobilization-based techniques. Manipulation-based techniques (e.g., translatory thrust) apply a high-velocity, low-amplitude force to the spine and are often accompanied by an audible sound from one or more joints. In contrast, mobilization-based techniques (e.g., muscle energy) use a low-velocity, low-force approach that generally does not produce audible joint sounds. While there is growing evidence for the clinical effectiveness of manipulative therapies to treat low back pain, little is known on the physiologic consequences and effects of either manipulation-based or mobilization-based treatments. Further, addition data are needed to understand how these different manual therapy techniques effect clinical changes in pain and disability. Accordingly, we propose an investigator-blinded, sham-controlled study to test the mechanisms and effectiveness of two manual therapy techniques applied to individuals with chronic low back pain. Specifically, the RELIEF Study (Researching the Effectiveness of Lumbar Interventions for Enhancing Function Study) is an exploratory Phase II randomized clinical trial (RCT) with a nested mechanistic design. As such, there are two major goals of this study. One is to determine the biological mechanisms of these two manual therapy interventions commonly used in the treatment of chronic low back pain. The second is to conduct an exploratory RCT determine the effectiveness of these two manual therapies at reducing pain and disability in patients with chronic low back pain compared to each other as well as a sham control group. Participants with chronic low back pain will be randomly assigned to one of the three treatment arms: 1) spinal manipulation, 2) spinal mobilization, or 3) sham ultrasound therapy (n=54/treatment arm). Treatments will be delivered twice per week for 3-weeks. Within each treatment arm, study participants will be randomly assigned to one of three physiological outcome groups to determine the muscular, spinal, and cortical effects of manual therapies. Treatment interventions will be delivered twice per week for 3-weeks. Participants will be assessed at baseline, immediately after their initial treatment intervention, 48 hours, and 1 month after the last treatment intervention. The proposed experiments will provide a systematic assessment of the muscular, spinal, and cortical mechanisms of two of the most popular manual therapies for the treatment of LBP within an exploratory Phase II RCT design.